This invention relates to nuclear reactors. It relates particularly to a grid structure used for supporting and retaining the fissionable fuel rods used in a nuclear reactor core.
Most nuclear reactors use as fuel, elongated rods of fissionable material arranged and supported in a spaced, parallel array between upper and lower core support plates. To provide integrity within the supports, the fuel rods are divided into groups and the fuel rods in each group are formed as an integral fuel rod assembly prior to placement between the reactor core support plates.
More specifically, the fuel rods in each group have been in the past typically arranged in spaced, parallel arrangement with each other in supporting and spacing frames or grids. The grids are formed of a plurality of interconnected metal grid straps to provide a structural network of interconnected open grid cells, similar to an "egg crate divider".
U.S. Pat. Nos. 3,379,617 and 3,379,619 issued Apr. 23, 1968 to Andrews et al. and assigned to the assignee of the present invention, describe several previously known designs for grid structures used to support and retain fuel rods and associated tubular fuel rod thimbles within selected cells or openings in the grid structure by resilient, spring-like retainers. U.S. Pat. No. 3,379,617 to Andrews et al., in particular, discloses using dimples and slotted projections to align and retain the fuel rod in the center of an open grid cell.
It has been observed in practice that the prior fuel rod grid support structures did not always hold the fuel rod in the desired alignment during service, especially during variations in the pressure or flow of the cooling water used to surround the fuel rod assemblies in the nuclear reactor core due to unbalanced forces and bending moments produced in the fuel assemblies. It certain situations, the pressure and flow of the cooling water could produce oscillatory motion of the fuel rod assemblies which not only caused a change in alignment but could easily cause a fretting wear of the protective cladding of the fuel rods.
Due to extended fuel service life, it is anticipated that the dimples and other spring-like retainers used in prior grid structures to retain and support the fuel rods in the grid cells are subject to fatigue cracking. Normally, replacement of the entire fuel assembly support structure is needed to prevent any possible fatigue failure of the fuel rod support systems. Such replacements are costly and require additional maintenance time during refueling operations to prevent possible fatigue failures.